Electric resistance welded pipes are generally manufactured by a process of forming a steel plate into a pipe shape and connecting both width direction end portions of the steel plate by welding while pressing the width direction end portions against each other. To obtain high welding quality in such a process of manufacturing electric resistance welded pipes, an electric resistance welded pipe is made into a product by: ultrasonic flaw detection on a welded seam portion by use of an ultrasonic flaw detection apparatus and annealing (a process of tempering the welded seam portion that has approached a quenching state by welding) of the welded seam portion with a seam annealer.
When ultrasonic flaw detection is performed on a welded seam portion, normally, after weld beads are cut or after a hydraulic test, angle beam testing in which ultrasonic wave signals are caused to be obliquely incident on the welded seam portion, is performed. In angle beam testing, ultrasonic flaw detection needs to be performed with a sensor head being positioned with respect to the welded seam portion such that the ultrasonic wave signals are incident on the welded seam portion, the sensor head including an ultrasonic probe. In particular, when focused ultrasonic wave signals are used, since focal depth of the ultrasonic wave signals becomes short, positioning thereof requires accuracy.
However, since electric resistance welded pipes are subject to various forces on manufacturing lines, their welded seam portions are not necessarily positioned on a central line of the sensor head, and may be displaced in a pipe circumference direction from the central line of the sensor head. Thus, techniques have been proposed, the techniques including: a seam position detection technique using reflected ultrasonic wave signals from a defect in a welded seam portion (see Japanese Patent Application Laid-open No. 2011-227060); and a technique of detecting a seam position from a temperature distribution obtained by imaging of a welded seam portion of an electric resistance welded pipe with an infrared camera, and further correcting a seam position by use of reflected ultrasonic wave signals from minute oxides, which are present in the welded seam portion and do not influence the quality (see Japanese Patent Application Laid-open No. 2009-222408).
However, in both of the techniques described in JP '060 and JP '408, the seam positions are detected based on the reflected ultrasonic wave signals from the minute oxides present in the welded seam portions and do not influence the quality, the reflected ultrasonic wave signals being obtained by electronic or mechanical scanning near the welded seam portions of the electric resistance welded pipes with ultrasonic wave signals in the pipe circumference direction. Therefore, according to the techniques described in JP '060 and JP '408, when the minute oxides are not present in the welded seam portions, the seam positions are not detectable.
It could therefore be helpful to provide an ultrasonic flaw detection apparatus and an ultrasonic flaw detection method enabling a seam position to be accurately detected and flaw detection on a welded seam portion to be accurately performed without reliance on reflected ultrasonic wave signals from minute oxides present in the welded seam portion.